The Catalogue of Radial Velocity Standard Stars for Gaia I. Pre-Launch

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La at Telescope Euler-Swiss the on hnti au.Ti aaou illkl eueu o oth for useful be likely will catalogue This value. this than ABSTRACT ions ony- ased 1 − µ and wn alets 20Suen,Switzerland Sauverny, 1290 Maillettes, t lc ue ase,910Muo,France Meudon, 92190 Janssen, Jules place 5 ot, .Crifo F. , 1 n-ae bevtoshsbe newysne20 omoni to 2006 since underway been has observations und-based as er fi agtsaiiycieino 0 s m 300 of criterion stability target a lfil ll n eakbylwRSsatro 3ms m 33 of scatter RMS low remarkably a and pcr rmteEOacie epoieama ailveloci radial mean a provide We archive. ESO the from spectra ob airtduigsal eeec tr nw nadva in known stars reference stable using calibrated be to s dLAMOST. nd − bu v ilo tr onto down stars million five about pcrlrne847 range spectral (2005). scien al. expected et Wilkinson the in described of are details RVS the More s from stars. yield multiple variable of the of characterization and dynamics and tems, and detection chemistry o the the Way, impact as Milky tremendous such a cases, have science will many survey spectroscopic large a Ca nt’pcrcpcPoesn’(aze l,21) sresp is 2011), Development al., its a et Analysis (Katz by specifically and Processing’ for Processing ’Spectrocopic more Data compensated Unit and Gaia be (DPAC), The can scale). Consortium RV scale the wavelength in the shift scale in velocity b shift radial would a the otherwise with which c degenerated point, for mathematically zero mandatory wavelength (2 are the al. standards ibrating et Jasniewicz velocity in radial explained instrumen As ground-based The calibrated. observations. self RVS be the dispersio will from wavelength derived RVS survey be the will spectro RAVE consequence, law calib field a wavelength the on-board As integral no source. with to an and tion slit is proven analysis entrance RVS no already RV with The graph has to 2006). range al., suited et spectral (Steinmetz well This very stars. be hot in lines bnacsfraottomlinsasdw to down stars million two about for abundances 1 e vrsvrlyas h bevtoswr oeo h echell the on done were observations The years. several over ii h V a eovn oe f150adcvr the covers and 11500 of power resolving a has RVS The 3 e esrdwt hs ntuet,atrhvn carefull having after instruments, those with measured ies etRceeu 51 ai,France Paris, 75014 fert-Rochereau, cs rpe,mn ie fio and iron of lines many triplet, .Udry S. , rmnshv enotie o h 40sas With stars. 1420 the for obtained been have urements 4 .Hestro D. , − dx0,France 05, edex 7 m hc nldstenear-infrared the includes which nm, 874 OHP), ff er − V 5 1 n .Katz D. and , he ude forty-three hundred Three . NARVAL ≃ − 1 3adio and iron and 13 α noecs,suggesting case, one in − lmns n Paschen and elements, nsrc criteria strict on d nteTélescope the on letagreement ellent rlarge-scale er 3 ⋆⋆ ⋆ V Geneva d

c c.The nce. o these tor ≃ S 2018 ESO α yin ty -elements 2 Such 12. y e 011), onsi- (i.e. ys- ra- al- ce n n 1 e - t C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia ble for establishing the requirements for this external calibration amplitude in our sample has been reduced because of all these and for the acquisition of the necessary auxiliary data selection criteria and a careful analysis of the RV measurements In principle asteroids are excellent sources for calibrating available for them so far. The HR diagram of the 1420 candi- the zero point because their radial velocity can be derived dates is shown in Fig. 1, as originally published in Crifo et al. 1 from celestial mechanics with uncertainties lower than 1 m s− . (2010). However, they cannot be the main reference sources for Gaia because they are not numerous enough in the appropriate magnitude range and their sky distribution is limited to the Ecliptic Plane. When we started to search for suitable RV standard stars six years ago, there was no existing dataset fulfilling the Gaia RVS requirementsin terms of sky coverage,magnitude range, number of targets, and stability. The only official list of standard stars was the one compiled by the IAU Commission 30, which did not turn out to be suitable for our purpose.We thus built a stellar grid consisting of 1420 FGKM stars in the magnitude range 6 < V < 11, with a homogeneous distribution on the sky. The selection process of that sample is fully explained in Crifo et al. (2010). We briefly summarize it in Section 2. The RVS calibration requires standard stars with much bet- 1 ter RV stability than the 1 km s− accuracy expected from their future measurements with RVS, with no drift until the end of the 1 mission (2018). The value of 300 m s− is adopted as the stability level to be checked.To qualify as a referencestar, each candidate has to be observed at least twice before launch and another time during the mission to verify its long-term stability. The RV Fig. 1. HR diagram of the 1420 selected stars, with a colour code measurements available to date are described in Section 3, and corresponding to their origin. their combination into a homogeneous scale in Section 4. We compare this new catalogue to other studies in Section 5. The stablest stars are presented in Sect. 6 as are considerations about variable stars. Future work is described in Sect. 7. 3. Radial velocity measurements 2. Radial velocity standard-star candidates The RV standards to be used for the RVS calibrations should not 1 The selection of stars suitable for building the reference grid exhibit variations larger than 300 m s− during the five years of for RVS has been carefully considered and several criteria de- the Gaia observations, between 2013 and 2018. Since the stan- fined (Crifo et al., 2010). The list of candidates was established dards need to be known beforelaunch, we extrapolatedthis crite- from three catalogues: “Radial velocities of 889 late-type stars” rion to the current period in order to perform a pre-qualification (Nideveret al., 2002), “Radial velocities for 6691 K and M of the candidates according to their stability during the time giants” (Famaey et al., 2005), and “The Geneva-Copenhagen span of our ground-based observations, between 1995 and 2012. Survey of Solar neighbourhood” (Nordström et al., 2004), com- Considering that the candidates already have a good observa- plemented with IAU standards (Udry et al., 1999). We selected tional history, our strategy of pre-qualifying them is (i) to obtain the stars having the best observational history in terms of con- at least two new measurements per star before the Gaia launch, sistency of radial velocity measurements over several years. In separated by more than one year, and (ii) to verify that the vari- 1 that way we could focus on stars more likely to be stable over ation in these new measurements does not exceed 300 m s− . time. Moreover, since all the candidates are in the HIPPARCOS Then supplementary ground-based observations will have to be Catalogue we were able to check their properties (photometry, done during the mission to verify the long-term stability of the spectral type, variability, multiplicity, etc.) in a homegeneous reference grid. way. Another important criterion was that each candidate had to Since the candidates are distributed all over the sky, ob- have no close neighbour within a circular region of 80′′-radius. servations in both the northern and southern hemispheres need This criterion was set to ensure that their future RVS spectra will to be performed with different instruments. An observing plan not be contaminated by another overlapping spectrum, 80′′ be- has been prepared under the umbrella of the GBOG (Ground ing the length of an RVS spectrum projected on the sky. This Based Observations for Gaia) working group in charge of ac- criterion was verified thanks to the USNO-B1 catalogue. These quiring auxiliary data for the Gaia processing. Eighty nights different criteria led to a preliminary list of 1420 candidates hav- have been obtained over six years for that programme on four ing a high probability of being RV-stable and well suited to be- high-resolution spectrographs: ELODIE then SOPHIE mounted on coming reference stars for the RVS. the 1.93-m telescope at Observatoire de Haute-Provence (OHP), All of the 1420 candidates have at least three previous NARVAL on the Télescope Bernard Lyot at Observatoire du Pic RV measurements in the above-mentioned catalogues; however, du Midi, and CORALIE on the Euler-Swiss telescope at La Silla. since they have been obtained during observational runs span- RV measurements were also retrieved from the public ning irregular time baselines, their stability has not yet been en- archives of SOPHIE and ELODIE at OHP (Moultaka et al., 2004), sured, particularly not until the end of the Gaia mission. The and of HARPS at the ESO Advanced Data Products database. 1 main cause of RV variability at the level of 300 m s− and above Many measurements are available in these archives, in partic- is binarity, but the risk of having binary stars with significant ular due to the various exoplanet-hunting programmes. The full

2 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

ELODIE archive was not public when the observations started in Table 1. Summary of RV measurements available as of 2006, and the volume of the other archives increased dramati- November 2012, average of individual RV errors, and epoch cally during the course of our programme. The recent query of range of the measurements. the archives allowed us to retrieve several thousands of relevant measurements. There were many time series available for some NRV N ǫ date ∗ 1 stars. For time series in one single night, only the observation m s− range with the highest signal-to-noise ratio was considered. Additional SOPHIE 2945 729 6.7 2006 - 2012 data from the private CORALIE archive have also been provided CORALIE 2470 775 12.9 1999 - 2012 by the Geneva Observatory. NARVAL 213 157 17.6 2007 - 2012 ELODIE 3931 372 17.4 1995 - 2006 SOPHIE, CORALIE, ELODIE and HARPS have respectively re- HARPS 655 113 2.2 2003 - 2009 solving powers of R = λ/∆λ, 75000, 50000, 42000, and 120000, respectively, and cover the∼ visible range. They all have similar automatic on-line data reduction software to derive the stars. One measurement is missing for only a few stars in the barycentric RV by cross-correlation of the spectra with a numer- southern hemisphere. ical mask (Baranne et al., 1996). The spectral type of the numer- Figure 3 shows the histogram of the time span between the ical mask is chosen to be the closest to the observed star. It can first and last measurements of each of the 1420 candidates. This be G2, K5, M4, and M5 for SOPHIE, G2, K5 and M2 for HARPS is 6.35 years on average (median 4.37 years), up to 17 years and CORALIE, F0 and K0 for ELODIE. thanks to the ELODIE archive. NARVAL has a resolving power of R 78 000. Since this intrument has not been built for RV measurements∼ but for po- larimetry, the on-line reduction software does not include the RV determination. We measured it by cross-correlating the observed spectra with the SOPHIE G2 mask. Our main interest in observing with NARVAL is that its spectral coverage includes the RVS range (847 874 nm), allowing us to investigate possible systematic differences− occurring when measuring the RV in the full visible range and in that narrow NIR spectral range. It is, however, beyond the scope of this paper to enter into these considerations. Errors on radial velocities come from the photon noise, the wavelength calibration and some possible instrumental or other effects, which are impossible to quantify. It is, however, important to estimate errors, noted ǫ in the following, for the weight- ing scheme adopted to combine measurements taken with different instruments and under various observing conditions. The photon-noise uncertainty is used, deduced from the width and contrast of the cross-correlation function (hereafter CCF) and from the signal-to-noise ratio of the spectrum (Baranne et al., 1996; Bouchy et al., 2005), and provided by the data reduction software. External systematic errors are then quadratically added 1 to the photon-noise uncertainty: 0.8 m s− for HARPS (F. Bouchy, 1 private communication),4 m s− for SOPHIE (Boisséet al., 2012), 1 5 m s− for CORALIE (D. Naef, private communication), and 15 1 m s− for ELODIE (Baranne et al., 1996). These systematic errors Fig. 3. Distribution of time baselines of observations for the have been estimated for the most precise observing mode of the 1420 candidates. instruments where the stellar spectrum is recorded simultane- . ously with a thorium-argon calibration. But not all the HARPS, SOPHIE, and ELODIE spectra in the sample presented here have been obtained in that mode, and the corresponding RV measurements have larger errors. This is taken into account by dou- bling the systematic error listed above for exposures without the 4. Combining RVs from different instruments simultaneous calibration. It is worth recalling that ǫ estimated that way does not represent the total RV uncertainty, but is only An important step in the process of determining whether a star meant to combine relative measurements with proper weights. has a stable radial velocity or not is to combine the measure- Table 1 summarizes the status of the observations, provid- ments obtained with the different spectrographs. This requires ing the number of spectroscopic measurements obtained so far taking the individual zero point of each instrument into ac- with each instrument (NRV), the corresponding number of stars count. The zero point is related to the instrument itself, its spec- (N ), the average errors of individual RV measurements ǫ, and tral range, and resolution, and to the calibration procedure and the∗ date range of the observations. In total there are 10214 RV the method used to derive an RV. In our case the same algo- measurements. rithm of cross-correlation with a numerical mask has been used Figure 2 represents the distribution of the 1420 candidate that should minimize the offsets between instruments. However, stars on the celestial sphere, with a colour code indicating the since different masks have sometimes been used, a dependency number of measurements obtained for each star. As expected, at of the offsets on the colour is expected. We adopted the SOPHIE least two measurements are available for the vast majority of the frame as the radial velocity reference scale for all spectrographs,

3 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

60°

30°

0° 18h 12h 6h 0h

-30°

-60°

Fig. 2. Distribution of the 1420 stars on the celestial sphere in equatorial coordinates. The colour code indicates the number of measurements obtained for each star. A dashed line indicates the projection of the Ecliptic plane, a dotted line that of the Galactic plane.

and we made certain to have a sufficient number of stars observed in common with SOPHIE and with the other instruments to derive the offsets. To measure the offset between SOPHIE and the other instruments, we first computed the weighted mean and standard deviation of the RV measurements per instrument and per star. The 2 weight of each individual measurement RVi is defined as 1/ǫi , ǫi being the RV error mentioned in Sect. 3. Figure 4 shows the RV offsets between the various instruments versus the B V colour index. The error bars, when available, are the quadratic− sum of the two standard deviations obtained for a given star observed several times with the two considered instruments. A large error bar is an indication that the star is probably variable. The offsets are constant in general, except for ELODIE where a dependency on colour is clearly seen. The scatter is also much higher when ELODIE is involved. The scatter reflects the RV variations, expected to be more pro- nounced in the case of a secular drift as the time baseline of the measurements increases, which is the case when ELODIE obser- Fig. 5. Offsets between RV measurements from HARPS and vations are considered. The scatter also reflects the RV errors, CORALIE showing a remarkably low RMS scatter. which are higher for ELODIE than for the other instruments (see Table 1). Table 2 summarizes the mean difference and RMS scatter obtained when comparing the measurements from two instruments. In this process, an iterative clipping at the 3σ level is performed to remove the outliers. The offset column provides the velocity corrections applied to the measurements of CORALIE, NARVAL, ELODIE, and HARPS to translate them onto the SOPHIE scale. – the basic data from Hipparcos: HIP number, equatorial coordinates, V magnitude, B V, spectral type; Boisséet al. (2012) have also measured the RV offset be- – the catalogue(s) from− which the star was selected : tween SOPHIE and ELODIE and found a steeper slope ( 425.6(B Nidever et al. (2002), Famaey et al. (2005), Nordström et al. V) + 202.4) but their fit is limited to B V < 0.75. Table− 2 and− (2004) or IAU standard (Udry et al., 1999); Fig. 5 also present the comparison of− the CORALIE and HARPS measurements as an illustration of the excellent agreement be- – the weighted mean radial velocity in the SOPHIE scale, RVS, tween those two instruments. with a weight wi applied to each individual velocity measurement RV being w = 1/ǫ 2 and ǫ the quadratic sum of Two tables are provided for the catalogue, only available i i i′ i′ ǫ and the offset error listed in Table 2; electronically. Table 3 gives the original individual RV measure- i – the internal error of RVS : I = wi ǫi′/ wi; ments, and Table 4 gives for each star : Pi Pi

4 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Table 2. RV oﬀsets between instruments derived for stars in common.

Instruments Ncommon N Offset ∗ 1 (a) (m s− ) CORALIE SOPHIE 164 154 -23 2.4 NARVAL −SOPHIE 114 108 -26 ± 6.2 ELODIE −SOPHIE 277 247 (b)± HARPS −SOPHIE 36 34 17 5.0 HARPS−CORALIE 88 84 31 ± 1.5 − ± Notes. (a) N is the number of stars effectively used after an iterative clipping at a∗ 3σ level. (b) In the case of ELODIE SOPHIE a linear regression was performed, giving an offset as a function− of the colour index Offset= 259.0(B 1 − − V) + 105.2 2.9 m s− . ±

– the RVS weighted standard deviation σRVS deﬁned as

wi 2 Pi 2 σ = wi(RVi RVS) RVS 2 2 X ( wi) wi i − Pi − Pi – the number of observations N; – the RVS uncertainty deﬁned as the maximum of the standard

error σRVS / √N and I/ √N (Jasniewicz & Mayor, 1988); – the time baseline in days; Fig. 6. Distribution of RV variations of the candidate standard – the mean Julian day of the N observations. stars having at least two RV measurements separated by 100 days or more. The variation is defined by 3σRVS . A dashed line 1 shows the 300 m s− stability threshold adopted for the calibration of the RVS instrument. Table 3. Excerpt of the catalogue table giving the 10214 original . individual RV measurements and their error ǫ (see Sect. 3), the Julian day of the observation, the instrument, and mask used. The name of the instrument is abbreviated with its first letter. the catalogue presented here have been selected from the stable ID Instr. JD RV ǫ mask stars of Nidever et al. (2002) and fulfil the Gaia-RVS require- 1 1 -2400000 km s− km s− ments. Our catalogue also has 355 stars in common with the HIP000296 C 52173 10.932 0.0070 G2 recent list of Chubak et al. (2012) which provides RVs for 2046 HIP000296 C 53262 10.930 0.0072 G2 nearby FGKM stars. Comparisons with those two catalogues are HIP000296 C 54354 10.952 0.0064 G2 shown in Fig. 7 as a function of B V. There is general good HIP000296 C 55167 10.948 0.0109 G2 ff− HIP000407 C 51542 12.346 0.0093 G2 agreement with small zero-point di erences. The outliers reveal HIP000407 C 52649 12.316 0.0069 G2 the intrinsic variable stars, as well as systematics due to method- ology. For B V & 1.2 there is a clear systematic effect. We note HIP000407 C 53921 12.328 0.0075 G2 − HIP000407 C 55167 12.331 0.0123 G2 that both Nidever et al. (2002) and Chubak et al. (2012) use the HIP000420 C 51382 34.861 0.0082 G2 same technique of chi-square minimization to determine the rel- HIP000420 C 53218 34.852 0.0083 G2 ative shifts between a template spectrum and an observed spec- HIP000420 C 55167 34.868 0.0130 G2 trum, but not in the same spectral range. They use an M-dwarf HIP000420 C 55758 34.879 0.0068 G2 observed spectrum as template for cool stars and a solar spec- HIP000466 S 55131 -8.371 0.0080 K5 trum for the other stars. The two catalogues are constructed on HIP000466 S 55452 -8.364 0.0080 K5 the same scale, as explained in Chubak et al. (2012). We divide HIP000556 C 55882 0.109 0.0110 G2 the comparison of their values to the present ones into two parts, HIP000556 C 56221 0.091 0.0135 G2 HIP000556 S 55129 0.102 0.0086 G2 according to their reference template. Results are presented in Table 5. We get very similar zero-point difference in both cases, Figure 6 shows the histogram of the variations of the 1420 which is expected since the catalogues of Nidever et al. (2002) candidates. We define the level of stability of a given star by and Chubaketal. (2012) are on the same scale. The very 1 3σRVS . We find that 92.8% of the stars have a stability better low RMS scatter of 33 m s− obtained for the comparison to 1 than300m s− , which is the threshold defined for the calibration Nidever et al. (2002) for FGK stars is remarkable for two rea- of the RVS instrument. More than 1000 stars exhibit a stability sons. First, since the median epoch of Nidever et al. observa- 1 1 better than 100 m s− (i.e. σRVS < 33ms− ). tions (1999.0) is earlier than ours (2006.9), it means that most of the stars in common have not varied much during that period. Second, it also shows that the precision of the present catalogue 5. Comparison with other catalogues 1 is not worse than 33 m s− . The same level of scatter has al- The catalogue of Nidever et al. (2002) contains 889 late-type ready been measured by Nidever et al. (2002) when they com- stars followed during several years, with 782 of them exhibit- pared their catalogue to the best set of standard stars available 1 ing a velocity scatter less than 100 m s− . Some 336 stars of at the time, assembled by the Geneva team (Udry et al., 1999)

5 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Table 5. Comparison of RVs obtained with by Nidever et al. (2002) and Chubak et al. (2012) and those presented here, according to the templates they used. Diﬀerences, ∆RV = RVS 1 − RVCHUBAK/NIDEVER (Fig. 7) and RMS scatter in m s− are derived with an iterative clipping at a 3σ level. N is the number of ∗ stars in common, No is the number of outliers removed.

source stat Sun template M-dwarf template N 326 29 N∗ 15 0 Chubak et al. o ∆RV 63 -98 RMS 100 182 N 308 28 N∗ 22 0 Nidever et al. o ∆RV 72 -141 RMS 33 178

and based on CORAVEL and ELODIE. The ELODIE measurements that were used to build the Udry et al. standard star catalogue are Fig. 8. Comparison of RVs obtained with by Lathamet al. ff among those retrieved from the archive for the present catalogue. (2002) and those presented here. The o set and RMS values We do not show the comparison to Udry et al. (1999) since we have been obtained after rejectiing two outliers. . have measurements in common. It is worth noticing that there is no correlation of the differences with Nidever et al. (2002) with B V in the FGK range, even though the stars of the present catalogue− have been analysed with different masks. For RVs obtained by Chubak et al. (2012) and Nidever et al. (2002) with an M-dwarf template, the mean difference and RMS are much larger. Figure 7 suggests that there might be a correlation of these differences with colour, but the B V interval is too narrow to confirm this claim. Such significant− offset and RMS compared with Udry et al. (1999) have already been found and discussed in both articles, so we confirm with a larger sample the assertion by Chubak et al. (2012) that 'the M dwarf velocities in general, from all surveys, remain uncertain at the level of 1 200 m s− (RMS) and harbor uncertain zero points at the level 1 of150ms− '. The comparisonwith the catalogue of Latham et al. (2002) is shown in Fig. 8. This catalogue contains 1359 single-lined stars on the Center for Astrophysics (CfA) scale. The difference is: 1 ∆RV = RVS RVLATHAM = 161 m s− for 66 stars in common after rejection− of two outliers. This offset is quite significant but 1 it agrees with the zero-point correction of 139 m s− that they Fig. 9. Difference in radial velocities of the Chubak et al. stan- determined from asteroids. Although the Latham et al. RVs are dard stars for 63 stars in common. The offset and RMS values 1 presented as having errors ranging between 0.5 to 1.5 km s− ,we have been obtained after rejecting seven outliers at B V > 1.2. 1 measure a much lower RMS scatter, 164 m s− , which suggests − that their errors are lower for the 65 common stars.

standards useful for many projects other than Gaia. Table 4 can 6. Stable and variable stars easily be queried with criteria such as Chubak et al. (2012) present a list of 131 FGKM standard stars – at least 4 measurements available, having constant radial velocity over ten years. We have 63 stars 1 – standard deviation on the SOPHIE scale σRVS < 33ms− (i.e. 1 in common with this list and the comparison is presented in stability better than 100 m s− ), 1 Fig. 9. The average offset is ∆RV = RVS RVChubak = 81ms− , – time baseline of ten years at least. 1 − with an RMS scatter of 63 m s− after rejection of seven outliers at B V 1.2. This RMS scatter is lower than in the compari- More than 300 stars verify these conditions and should son with− the main catalogue of Chubak et al. (2012) presented be followed up in the future to become official RV-standards. 1 in Table 5, but does not reach the very low value of 33 m s− ob- Several of these stars are shown in Fig. 10. They were observed tained in the comparison with Nidever et al. (2002). This means with different instruments and the agreement of the measure- that either the Chubak et al. standard stars are less stable than the ments demonstrates that the zero points have been properly cor- Nidever et al. ones, or that their errors are significantly higher rected. than the Nidever et al. ones. Several stars showing a remarkable trend in their RV are The catalogue presented here is also a good source for very shown in Fig. 11. The presence of a low-mass companion stable stars that are good candidates for becoming official RV- likely explains these drifts. The oscillation of HIP017960 is

6 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia due to a Jupiter like exoplanet already reported by Boisséet al. (2012). HIP079248 is also known to harbour two exoplanets (Go´zdziewski et al., 2006). A Keplerian orbit with a period of 1161 45 days was fitted on HIP098714 measurements by Sivan± et al. (2004), revealing the presence of a planetary com- 1 panion. Its semi-amplitude of 95 17ms− is still low enough to make this star suitable for the RVS± calibrations. Interestingly, HIP041844 and HIP044089 are part of the Nidever et al. stable stars. The drift of these two stars is quite slow, which might ex- plain why Nidever et al. have not been able to detect it within the time range of their observations. This demonstrates that it is very important to have observations over a sufficient time base to detect such variations. However,the numberof binary stars remain- ing in our sample is expected to be very low. The selection criteria described in Crifo et al. (2010) and recalled in Sect. 2 elimi- nated all known double stars and variable stars (photometrically and spectroscopically), as well as those with fainter neighbours. HIPPARCOS efficiently detected double stars with separations of 0.1′′ to 10′′, and magnitude differences lower than 4 magni- tudes (see Fig. 3.2.106 available in the HIPPARCOS Catalogue2 Fig. 12. HR diagram of the 1420 candidates with a colour code volume 1 ). Such stars are labelled in HIPPARCOS and were corresponding to their stability level 3σRVS . The less stable stars, not selected by Crifo et al. (2010). This guarantees that there is 1 with 3σRVS > 300 m s− , are indicated as black points. Those no spectroscopic binaries with short-term variations in our sam- are not suitable for calibrating the RVS instrument. The HR dia- ple, because the candidates are very nearby stars, all closer than gram is divided into four parts correspondingroughly to turn-off, 150pc except some giants. Only a few wide systems with a faint main sequence, subgiants, and giants where median RV scatter, secondary (M star or white dwarf) and a rather long period may and the fractions of stable and variable stars have been evaluated therefore remain in the sample. For those, the second round of (Table 6). observations during the Gaia mission will be decisive in con- straining their long-term behaviour. With new observationstobe done after 2014, we will extend the time baselines by several 1 Table 6. Percentage of stable (σRVS < 30 m s− ) and variable years (current median epoch of our observations is 2006.9), and 1 (σRVS > 100 m s− ) stars among our 1420 selected candidates, we will have at least three measurements per star (67% of the in four parts of the HR diagram. stars have already 3 measurements or more), well distributed in time and with a much higher precision than the threshold of 300 HR part N median σRVS stable variable 1 1 m s− . This will guarantee that we cannot miss any long period m s− % % binary with large amplitude that would not be suitable for the main sequence 397 17.0 76 9 RVS calibrations. turn-off 662 18.3 74 6 It is interesting to investigate whether the most stable stars, subgiants 199 17.7 75 5 1 giants 154 34.6 46 11 with σRVS < 30 m s− , or on the contrary the less stable stars, 1 with σRVS > 100 m s− , display in peculiar locations of the HR diagram. Figure 12 shows the HR diagram of the 1420 candi- i dates with a colour code corresponding to their stability level for an Fe line in a K giant (Chiavassa et al., 2011). Other astrophysical processes may affect the spectroscopic RV of a star, 3σRVS . The median σRVS and the fraction of stable and non- stable stars are evaluated into four parts of HR diagram, as in- such as its rotation and activity, the granulation, or the presence of low-mass companions. dicated in Fig. 12 : main sequence (MV > 4.5, B V > 0.65), − One of the objectives of our ongoing project will be to pro- turn-off (B V 0.65), giants (MV 2.5, B V > 0.85), and subgiants (the− rest≤ of the sample). Results≤ of− the counts are given vide an accurate kinematical RV for each of the 1420 stars in in Table 6. They clearly show that giants are less stable on av- the sample. This will be done by deriving the gravitational red- erage than dwarfs and subgiants, confirming previous findings shift from atmospheric parameters and the convective shift cor- (Döllinger et al., 2005). Not only is the presence of a stellar or rections from three-dimensional hydrodynamical model atmo- substellar companion orbiting around the giant star suspected to spheres. Another step will be to determine the zero point of the cause the RV variations, but also the possible rotational modula- catalogue by comparing the RV measurements of a selection of tion of surface inhomogeneities or pulsations. asteroids with their kinematical velocities as derived from celestial mechanics. Finally, we will continue the observations of the 1420 candidate stars during the Gaia mission in order to validate 7. Next steps the long-term stability of their RV. All those steps are essential for calibrating the measurements that will be performed with the There are many physical mechanisms that affect spectroscopic Gaia-RVS. radial velocities (Lindegren & Dravins, 2003). For instance, the convective shifts caused by motions in stellar atmospheres de- pend on stellar lines and on the temperature and gravity. They 8. Conclusion can reach3 kms 1 fora Caii line in an F dwarf, and 0.4kms 1 − − − We have presented the pre-launch version of the catalogue of 2 on-line at RV standard stars for Gaia, assembled thanks to a long-term ob- www.rssd.esa.int/index.php?project=HIPPARCOS&page=Overview serving programme started in 2006 on several spectrographs and

7 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia with archived measurements. The precision of this catalogue is 1 at the same level as that of Nidever et al. (2002), 33ms− , for FGK stars (0.5 . B V . 1.2), showing that the∼ RV measurements from the diﬀerent− instruments have been properly combined. The vast majority of the 1420 selected candidates are 1 found to be stable at the 300 m s− level, which makes them suitable for calibrating the RVS instrument. Their long-term stability will have to be conﬁrmed with new ground-based observations during the Gaia operations. The stable stars presented here have characteristics that make them very useful as standards for many projects other than Gaia. They have a good coverage of important parameters, such as sky distribution, apparent magnitude, spectral type, and luminosity class. They can be used to intercompare intruments and calibrations of zero point of spectroscopic observations. The stars and measurements of this list and those of all other studies on RV- calibrators will soon be discussed within the task group ’radial- velocity standard stars’ of the IAU Commission 30.

Acknowledgements. We are indebted to AS-Gaia, PNPS, and PNCG for their financial support of the observing campaigns and help in this project. We thank Frédéric Arenou for valuable discussions about the contamination of our sample by binary stars. We warmly thank Sergio Ilovaisky, Maxime Marmier, and Dominique Naef for helping us retrieve relevant data in the OHP and Geneva archives. Many thanks also go to staff of the observatories who made observations, and to Lionel Veltz who also contributed to them. We also thank the staff maintaining the public archives of ready-to-use spectra at OHP and ESO. LC acknowledges a financial support from CNES.

References Baranne, A., Queloz, D., Mayor, M., et al. 1996, A&AS, 119, 373 Boissé, I., Pepe, F., Perrier, C., et al. 2012, A&A, 545, A55 Bouchy, F., Pont, F., Melo, C., et al. 2005, A&A, 431, 1105 Chiavassa, A., Bigot, L., Thévenin, F., et al. 2011, Journal of Physics Conference Series, 328, 012012 Chubak, C., Marcy, G., Fischer, D. A., et al. 2012, arXiv:1207.6212 Crifo F., Jasniewicz G., Soubiran C., et al., 2010, A&A, 524, A10 de Bruijne, J. H. J. 2012, Ap&SS, 341, 31 Döllinger, M. P., Pasquini, L., Hatzes, A. P., et al. 2005, The Messenger, 122, 39 Famaey, B., Jorissen, A., Luri, X., et al. 2005, A&A, 430, 165 Go´zdziewski, K., Konacki, M., & Maciejewski, A. J. 2006, ApJ, 645, 688 Jasniewicz, G., & Mayor, M. 1988, A&A, 203, 329 Jasniewicz, G., Crifo, F., Soubiran, C., et al. 2011, EAS Publications Series, 45, 195 Katz, D. 2009, SF2A-2009 Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics, 57 Katz, D., Cropper, M., Meynadier, F., et al. 2011, EAS Publications Series, 45, 189 Katz, D., Munari, U., Cropper, M., et al. 2004, MNRAS, 354, 1223 Latham, D. W., Stefanik, R. P., Torres, G., et al. 2002, AJ, 124, 1144 Lindegren, L. & Dravins, D. 2003, A&A, 401, 1185 Moultaka, J., Ilovaisky, S. A., Prugniel, P., & Soubiran, C. 2004, PASP, 116, 693 Nidever, D. L., Marcy, G. W., Butler, R. P., Fischer, D. A., & Vogt, S. S. 2002, ApJS, 141, 503 Nordström, B., Mayor, M., Andersen, J., et al., 2004, A&A, 418, 989 Sivan, J.-P., Mayor, M., Naef, D., et al. 2004, Planetary Systems in the Universe, 202, 124 Steinmetz, M., Zwitter, T., Siebert, A., et al. 2006, AJ, 132, 1645 Udry, S., Mayor, M., & Queloz, D. 1999, IAU Colloq. 170: Precise Stellar Radial Velocities, 185, 367 Wilkinson M., et al., 2005, MNRAS, 359, 1306

8 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Fig. 4. Oﬀsets between RV measurements from SOPHIE and the other instruments as provided in Table 2. Error bars when available, are the quadratic sum of the two standard deviations obtained for a given star observed several times with the two instruments being considered.

Table 4. Excerpt of the catalogue table giving the mean RV on the SOPHIE scale of the 1420 standard star candidates (see text for the description of the columns). Abbreviations are used for the catalogue(s) from which the star was selected: ni=Nidever et al. (2002), fa=Famaey et al. (2005), nor=Nordstr¨om et al. (2004) in column 6.

ID α, δ Vmag B V ST cat RVS I σRVS N uncertainty span mean JD − 1 1 1 1 km s− km s− km s− km s− days - 2400000 HIP000296 00:03:41.30 -28:23:45.1 8.24 0.780 G8V nor 10.963 0.0078 0.0117 4 0.0059 2994 53739 HIP000407 00:04:58.63 -70:12:43.9 8.13 0.710 G5V nor 12.351 0.0086 0.0130 4 0.0065 3625 53319 HIP000420 00:05:07.60 -52:09:05.1 7.53 0.577 G0V nor 34.889 0.0086 0.0129 4 0.0065 4376 53881 HIP000466 00:05:34.91 +53:10:18.1 7.23 1.174 K0 fa -8.367 0.0080 0.0049 2 0.0057 321 55291 HIP000556 00:06:46.94 -04:20:59.3 8.21 0.574 F8 nor 0.113 0.0104 0.0165 3 0.0095 1092 55744 HIP000616 00:07:32.34 -23:49:08.2 8.70 0.798 K0V ni -42.994 0.0086 0.0049 4 0.0043 3380 53629 HIP000624 00:07:37.41 -45:07:09.5 8.19 0.584 F8/G0V nor 14.802 0.0117 0.0134 2 0.0095 725 55145 HIP000699 00:08:41.02 +36:37:38.7 6.21 0.504 F8IV ninor -15.116 0.0043 0.0119 16 0.0030 5160 54912

9 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Fig. 7. Diﬀerence between our radial velocities and those from Nidever et al. (2002) (left panel) and Chubak et al. (2012) (right panel) as a function of the B V colour index. Red dots indicate the stars they have analysed with an M template, while for the other ones they used the solar spectrum− as reference. The oﬀset and RMS values are given for FGK stars and for M stars in Table 5.

10 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Fig. 10. RV measurements for some of the long-term stable stars of the catalogue. The RV axis is centred on RVS and spans 300 1 m s− . The shaded area represents 1-σRVS and the dotted lines 3-σRVS . Blue dots represent SOPHIE mesurements, green stars CORALIE, pink squares ELODIE, purple triangles HARPS, red diamonds NARVAL. 11 C. Soubiran et al.: The catalogue of radial velocity standard stars for Gaia

Fig. 11. Trend in RV measurements for some stars of the catalogue. Symbols as in Fig. 10 .